Knowledge of earth formation elemental composition is useful in a wide variety of fields including mining, hydrology, geology, and hydrocarbon production. More specifically, elemental compositions of elements in formations penetrated by a borehole are used to determine a wide range of physical, lithologic and fluid saturation properties of the formation. The term “elemental composition” used in this disclosure refers to both the detection of the presence of an element and also to the measure of the amount or “concentration” of an element in the formation.
Several examples of uses of elemental composition measurements are listed below. These examples are by no means intended to be an all-inclusive list of uses of elemental composition measurements in earth formations. Detection of silicon (Si) can indicate that the formation is sandstone or shale. Detection of calcium (Ca) can indicate that the formation is limestone or dolomite (carbonates). Detection of magnesium (Mg) can indicate that the formation is dolomite. Detection of chlorine (Cl) can indicate that the formation is saturated with saline water, since significant amounts of chlorine are usually not found in common rock matrices such as sandstone, limestone and dolomite. A measure of hydrogen content can be used to determine formation porosity, since major concentrations of hydrogen are found in fluid saturating the formation rather than in the formation matrix.
Elemental composition can be measured using a number of techniques. Element concentration measurement techniques applicable to a borehole environment are much more limited.
One borehole technique comprises irradiating a formation with a source of gamma radiation disposed within a tool conveyed within the borehole, and measuring the intensity of low energy radiation back-scattered by the borehole environs into the tool using a gamma ray detector. Back-scattered radiation is measured in a relatively low energy range of the gamma ray spectrum dominated by the photoelectric effect. This photoelectric, or “Pe” radiation, can be related to formation elemental composition. Since the energy of gamma radiation is relatively low, the measurement is adversely affected by the near borehole environs including drilling fluid or “mud” within the borehole, and also by the structure of the tool. Pe measurements are severely degraded in boreholes drilled with heavy drilling muds weighted with barite or other materials with large atomic weights.
A second borehole elemental composition measurement technique comprises irradiating a formation with a source of neutrons disposed within a tool conveyed within the borehole, and measuring the intensity and energy of induced gamma radiation using a gamma ray detector disposed within the tool. If an isotopic or “chemical” source of neutrons is used, such as a source comprising a mixture of americium (Am) and beryllium (Be), most measured gamma radiation results from the capture of thermal neutrons by elements in the formation and borehole environs. Furthermore, individual nuclei, upon capture of thermal neutrons, emit gamma radiation at characteristic energies and at characteristic relative intensities. A measure of energy and relative intensity of capture radiation, commonly referred to as a capture gamma ray “spectrum”, can be used to identify the presence of certain elements. A measure of the intensity of the spectrum from a given element can be used to determine the concentration of that element. Ratios of characteristic energies can be indicative of relative concentrations of elements. Thermal capture radiation energies for many common elements in earth formations are significantly greater than the previously discussed photoelectric energy range. Thermal capture radiations at higher energies are, therefore, less adversely affected by the absorptive properties of the near borehole environs including the borehole mud and the tool. The tool, however, does present another problem in elemental composition measurements using thermal capture gamma radiation. Most tools comprise a significant amount of steel, especially in the pressure housing which protects the internal components of the tool from the harsh borehole environment. Iron (Fe) contained in steel produces capture gamma radiation with energies and intensities that interfere with capture gamma radiation from elements of interest.
Borehole tools, which are used to measure “logs” of parameters of interest as a function of depth within the borehole, typically fall into two categories. The first category is “wireline” tools wherein a “logging” tool is conveyed along a borehole after the borehole has been drilled. Conveyance is provided by a wireline with one end attached to the tool and a second end attached to a winch assembly at the surface of the earth. The second category is logging-while-drilling or “LWD” tools, wherein the logging tool is conveyed along the borehole by a drill string, and measurements are made with the tool while the borehole is being drilled. Steel in wireline logging tools produces interfering capture gamma radiation from Fe, but spectral processing can be used with good results to separate the Fe “noise” from induced radiation in Ca, Si, H, Cl and the like. LWD tools are usually disposed in the walls of a drill collar in the drill string. The drill collar wall serves as a pressure housing for the tool. Walls of the drill collar are steel and are typically several inches thick. Steel pressure housings of wireline tools are typically at least an order of magnitude thinner. There is, therefore, much more iron surrounding an LWD tool than surrounding a wireline logging tool. Since elemental spectral intensity increases as a function of element concentration for a given tool configuration and neutron source strength, interference from Fe in LWD tools is at least an order of magnitude greater than interference from Fe in wireline tool.
Gamma ray spectroscopy measurements made with a gamma ray detector imbedded within a collar of an LWD tool are typically fatally flawed by intense interference from gamma radiation resulting from thermal neutron capture in Fe. Furthermore, Pe measurements made with a gamma ray detector imbedded within the collar of an LWD tool is typically fatally flawed by excessive absorption of gamma radiation in the photoelectric range by Fe.